One known method for measuring a distance against a remote object is by using an optical reflective sensor based on the triangulation principle. A wide variety of applications have been derived from the method including semiconductor assembly equipment, manufacturing process control, liquid level monitoring, camera, ranger finder, and 3D profile measurement.
A conventional optical reflective sensor generally includes an optical emitter or light source for emitting an optical wave such as infrared light, an optical sensor for receiving the optical wave reflected by a remote object, and an electronic circuit for processing the wave signal from the optical sensor and may be in communication with other components in the system, such as a microprocessor or an ASIC (Application Specific Integrated Circuit). The optical emitter, optical sensor and the electronic circuit are normally packaged and manufactured into a single electronic assembly or a module.
Designs of mechanical structures for optical reflective sensor assembly have been developed to achieve higher performance. For example, the optical sensor may be oriented at a slant position angularly offset from the optical axis of the emitted light such that the optical sensor is aligned with the image plane. Accordingly, more sensitive and accurate measurement can be performed when the image on the optical sensor is in focus.
Like many other electronic devices, shrinkage in the assembly size is preferred for the distance sensor to accomplish lower materials cost and portability of the final product. In specific manufacturing industries, such as the semiconductor packaging industry, where on-line monitoring is required, there is a strong need for short-range distance sensors with high accuracy to be mounted in most assembly machines under the constraints of limiting space, and light weight to enable fast-moving of assembly machine parts. Furthermore, there is also a large quantity of applications for miniature distance sensors in consumer electronics. For example, by incorporating a range-finder consisting of miniature distance sensor in a cell-phone, a device so equipped becomes capable of switching to speaker mode from handset mode when sensing the cell-phone being away from user for a certain distance. This feature is particular advantageous for users who need to use the cell-phone often during driving.
However, known manufacturing processes for distance sensor assemblies are performed at the component level, limiting the further shrinkage of distance sensor size. This is because the assembly equipment in the packaging industry works on a planar platform with standard accessories in order to avoid the problems of wire-bonding on slant surfaces. Dedicated and hence costly equipment is required for making miniature assembly with the foregoing slant sensor arrangement, for example, below the size of about 2 mm×2 mm. As a result, the manufacturing cost for small size optical distance sensors becomes undesirably high.
U.S. Pat. No. 6,624,899 entitled “Triangulation displacement sensor” describes an optical sensor based on a triangulation method. The beam shaping elements are arranged such that the image formed on the detector element is slightly homogenized. As a result, the intensity of the image has smooth peaks and symmetrical variation.
US Patent Application Publication Number US2004/066499A1 entitled “Optical distance measuring device” discloses an optical distance detecting device having two detectors. One of the detectors is inclined at an angle between 10° and 170° with respect to the reference plane of the emitting axis in order to improve the result of measurement and to realize background suppression sensors having small dimensions.
JP Patent Application of publication number JP2006134922 entitled “Lead frame and physical value sensor using this, and further, method of packaging physical value sensor chip” discloses a lead frame structure which improves the yield of chip mounting at an inclined surface.
While the foregoing examples suggest slant arrangement of sensors, none of them provide a cost effective solution for assembling the slant structure.
Another problem for manufacturing miniature distance sensor relates to calibration. Sensing distances using small-sized photodiodes and position sensing detectors (PSD) are based on the magnitude of the photo-current generated by the light that is reflected from the objects being detected. The photo-current magnitude depends on the reflectivity of the object. Accordingly, there exists discrepancy in distance detection for objects with different reflectivity. To tackle this problem, more expensive detectors such as CCD/CMOS sensors are used to detect the actual light spot location on the sensor in order to provide high sensing accuracy and high tolerance against variation of object reflectivity. However, this approach requires complex lens structure and signal processing electronics to accomplish the whole sensing algorithm. This substantially increases the device size and the manufacturing cost. The object reflectivity issue discourages people from using the low cost and small size photodiodes and position sensing detector (PSD) for distance sensing applications. Therefore, the art would benefit greatly from cost-effective calibration methods for distance sensors using photodiodes and PSD which perform routine measurement on limited types of materials, such as for distance sensors in semiconductor packaging machines. The art would further benefit from improved optoelectronic distance sensors containing slant mounting surfaces and methods for manufacturing and calibrating the same that may be utilized in miniature assembly while keeping a low production and maintenance cost.